Eyes evolved to detect the directions from which ambient luminance contrasts emanate. With
varying degrees of acuity, both compound and simple chambered eyes register the angular
position of visual features relative to either the fovea (azimuth and elevation) or to other visual
features in the scene. Optic flow (dynamical changes in this angular information) is sufficient to
specify unit-less aspects of the spatial visual world such as 3-dimensional form (à la the kinetic
depth effect) or depth order (via dynamic occlusion). However, other aspects of the visual
world, such as perceived size and extent, require a combining of visual angles with appropriate
metrics, and in many cases, the body provides these requisite perceptual rulers. For example, via
a simple trigonometric formula, the angular elevation of visual features viewed on the ground
plane specifies their distance from the observer in units of eye-height.
The body provides a plethora of perceptual rules, each specific to the ongoing activity of the
perceiver. For example, the sizes of graspable objects are perceived relative to the size of one’s
hand, whereas the distance across an open field is scaled by how much walking will be required
to traverse it. The literature on perceptual body scaling is divided into three general classes.
Morphological scaling uses the metrics of the body’s skeletal structure—eye height, arm’s
length, hand size—and its extension through the wielding of tools. Physiological scaling draws
on the body’s internal, often metabolic, states as they relate to intended actions. For example,
the amount of walking required to traverse an extent is scaled by the bioenergetic costs of
locomotion. Finally, behavioral scaling is provided by the efficacy of intended actions.
Research on embodied perception addresses the following question: By what units are metric
spatial perceptions scaled? As summarized in this chapter, the solution to this problem is a
specification of what aspects of the body are used to scale spatial percepts given the observer’s
phenotype and the intended actions afforded by their physical surroundings. Roughly 100
research articles are reviewed.